Member Avatar for Straightone

Okay I am having trouble changing the inputted roman numeral into arabic number so I can add them up what am I doing wrong?
I know I still have a lot to go but I cannot even get the first steps right.

I am new to python.... so the hard coding I have seen so far is not helping and most coding is from Arabic to roman but i need from Roman to Arabic.

from string import *

"I" == 1
"V" == 5
"X" == 10
"L" == 50
"C" == 100
"D" == 500
"M" == 1000


roman = upper(raw_input("Enter the roman numeral to convert to arabic: "))

"I" == 1
"V" == 5
"X" == 10
"L" == 50
"C" == 100
"D" == 500
"M" == 1000

if roman[0] > roman[1]:
    arabic = roman[0] + roman[1]
    

print "The roman numeral",roman, "Is equal to",arabic
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  • Latest Post Latest Post by GrimJack

Dani AI

Generated

You are close. Two gotchas are tripping you up: (1) you are comparing characters like 'I' and 'V' instead of their numeric values, and (2) indexing with roman[0] and roman[1] will break for a 1-letter input. Building on ’s dict idea and ’s subtraction note, a simple, safe approach is to scan from right to left. If a symbol is smaller than the one to its right, subtract it; otherwise add it. Compare numbers, not letters, and do a little validation so junk like IL is rejected.

def roman_to_int(s):
    vals = dict(zip('IVXLCDM', (1,5,10,50,100,500,1000)))
    allowed = {('I','V'),('I','X'),('X','L'),('X','C'),('C','D'),('C','M')}
    s = s.strip().upper()
    if not s: raise ValueError('empty numeral')
    total = 0; prev = 0; last = ''; reps = 0
    for ch in reversed(s):
        if ch not in vals: raise ValueError('invalid char: %s' % ch)
        v = vals[ch]
        if v < prev:
            if (ch, last) not in allowed: raise ValueError('bad pair: %s%s' % (ch, last))
            total -= v; reps = 0
        else:
            total += v
            if ch == last:
                reps += 1
                if ch in 'VLD' or reps >= 3: raise ValueError('too many %s' % ch)
            else:
                reps = 0
        prev, last = v, ch
    return total

# Python 2 usage:
r = raw_input('Enter the Roman numeral: ')
try:
    print '%s = %d' % (r.upper(), roman_to_int(r))
except ValueError as e:
    print 'Error:', e

Notes:

  • Drop from string import *; use s.upper() directly.
  • Do not write "I" == 1; use the mapping above and always add/subtract integers.
  • If you still see a string, you are printing strings, not numbers. Format with %d (Py2) or print('{} = {}'.format(...)) in Py3.

Recommended Answers

All 5 Replies

Member Avatar for shadwickman

To start, '==' is not how you assign a value to a variable. Use '=' for that, as '==' checks for equality. Change those first few lines that have that (the ones assigning the letter to decimal values). And, you can't assign a int value to a str value. You need to use a variable, like this:

valueI = 1
valueV = 5

A better way of doing this is using a dictionary:

mydict = { 'I': 1, 'V': 5, 'X': 10, 'L': 50, 'C': 100, 'D': 500, 'M': 1000 }
# so, mydict['I'] contains the value of 1, and so forth.

Also, 'upper()' is built-in to the str object, so you need to call it by saying

roman = raw_input("Enter the roman numeral to convert to arabic: ").upper()

Try making those changes and then proceeding with the code. Good luck!

Member Avatar for GrimJack

Just being my pedantic self, you are changing inputted Roman Numerals to Hindi numbers- the Arabs borrowed the numbers from India (in Arabic they are known as 'Indian numbers' - arqa-m hindiyyah)

Member Avatar for Straightone

Just being my pedantic self, you are changing inputted Roman Numerals to Hindi numbers- the Arabs borrowed the numbers from India (in Arabic they are known as 'Indian numbers' - arqa-m hindiyyah)

I keep trying and trying but I just cannot get the answer to spit out in integers instead of a string...what am i doing wrong>??

Member Avatar for shadwickman

Well, int(arabic) should return the numeric value of the str arabic. If that's any help.

Member Avatar for GrimJack

Do not forget that anywhere in the string that if a numeral is smaller than the one that comes next, it is subtracted from the the one that comes next:

#
if roman[0] >= roman[1]:
#
arabic = roman[0] + roman[1]
Else
arabic = roman[1] - roman[0]

Or something like that.

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